Mobile communication systems have been developed with the aim to facilitate communication everywhere, with everyone and at any time. In recent years, mobile communication systems, and particularly cellular communication systems, experienced a huge increase, both in the number of users and in the quality and demands of services offered.
Commonly used and widely spread mobile communication systems such as the pan-European Global System for Mobile Communication (GSM) are cellular systems. A cellular system or network is characterized in that it is organized on a cell basis, wherein each cell comprises a base station whose radio coverage area defines the geographical spreading of this cell.
Since only a limited frequency band is available for an entire mobile communication network and each communication channel requires a certain bandwidth, it is essential to exploit the available frequency band as efficient as possible such that as many users as possible can be serviced in the network. Therefore, in a cellular network, the available frequencies are usually reused on a cell basis. This means that the same set of frequencies, i.e. the same frequency band, which is used in one cell are also used in another cell of the same system in order to increase the user capacity of the system. However, in this regard, there exists a drawback in that interferences between the communications of users in different cells may occur, when the same frequencies are used. Such interferences are desired to be avoided since the communication quality is deteriorated due to them. Thus, the same frequencies are to be reused only in cells which are spaced at a minimum distance from each other. This distance is usually called spatial frequency reuse D (see FIG. 1). However, the larger distance D is and, thus, the lower potential interferences are, the fewer users can be serviced in the system, i.e. the lower the capacity of the system is. That is the spatial frequency reuse D is desired to be as small as possible, in particular in view of an increasing number of users.
In FIG. 1, a cell structure of a cellular mobile communication system is depicted according to the hexagon model, i.e. each cell is illustrated as a hexagon, irrespective of its actual physical shape. The number in each cell denotes a certain set of frequencies, i.e. a certain frequency band, which is allocated to this cell. According to FIG. 1, three frequency bands 1, 2, 3 are exemplarily used and a cell using frequency band 1 is surrounded by cells using frequency bands 2 and 3. This results in a frequency reuse factor k being 3.
Within one cell communications are effected in so-called channels. In the GSM system, for example, the channel allocation comprises a segmentation of channels both in the frequency domain and in the time domain. Thus, by dividing the available frequencies in an uplink band (for communication between a mobile station and a base station) and a downlink band (for communication between a base station and a mobile station) a frequency division duplex (FDD) technique is implemented. Further, by dividing an uplink/downlink band in a time frame structure comprising time slots, a time division duplex (TDD) technique is implemented. Other communication systems may use one of these techniques, either FDD or TDD, or a combination of these.
In order to cope with the increasing requirements mentioned above in terms of number of users and demands of services, which are posed on mobile communications, mobile systems and networks of the third generation (3G) and even the fourth generation (4G) are under development and partly already in operation, e.g. the General Packet Radio Service (GPRS) and the Universal Mobile Telecommunication System (UMTS).
The current working assumption for a future communication system of a 4G cellular system in a high frequency bandwidth requirement amounts to 1 Gbps (Gigabits per second) in maximum data rate. To achieve reasonable multi-operator scenarios in view of suchlike requirements and with limited total bandwidth availability, the frequency reuse factor in the network must be low.
A method for channel allocation utilizing power restrictions is presented in U.S. Pat. No. 6,259,685. In this method the time-slotted transmissions of synchronized base stations are arranged in such a way that transmissions utilizing maximum power P do not occur at the same time t in cells sharing the same frequency band.
The principle of a time-slotted transmission power scheme according to the cited prior art solution is illustrated in FIG. 2. The figure shows the power restrictions of the base station for a situation of three neighbouring cells, with P denoting the transmission power of the base station of the respective cell and t denoting the time. In a normal situation, the single timeslots are allocated to different terminals at different geographical locations.
Another prior art method implementing power control is known as the Flarion Flexband (FF). According to this method sub-carriers are divided into three frequency groups. These three frequency groups are reused in neighbouring cells and the transmission power of each group is predetermined such that the same frequency is used at different powers in different cells. For example, one cell will use a frequency f1 at a high power while another neighbour cell will use the same frequency f1 at a low power. Through this method, the interference is suppressed.
According to another method in the prior art PCT/IB2005/000137 a predetermined power sequence may be used that defines the transmission powers for sub carrier and each time slot in a transmission frame. The power sequences of the cells are organised in order to reduce interference.
Due to the mobility of terminals and multi-media traffic, the traffic in the wireless systems is not always uniformly distributed in every cell. In other words, the amount of traffic in some cells can be extremely high during the specific period (e.g., rush hours). In the packet-based wireless systems, the cells with high loading may experience the congestion problem, namely, packets waited for a long time period in the queues for scheduling or packets even discarded due to either the buffer overflow or the queuing time over some threshold values.
Since the prior art methods mentioned above employ predetermined power sequences, as such the systems cannot adapt to load imbalance between cells since the predetermined power sequences assume uniform traffic distribution in the network.
The prior art mentioned above also suffers from the disadvantage that the methods employ the use of complicated power sequence designs that require additional pre-configuration work in the network planning phase.
Thus, a solution to the above problems and drawbacks is desirable for a cellular communication network, in which frequency reuse possibilities are limited.
Consequently it is an object of the present invention to alleviate the above drawbacks inherent to the prior art and to provide a method of controlling interference in a communication system.